A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. The major source for these materials in present day refining is the steam cracking of petroleum feeds. For various reasons including geographical, economic, political and diminished supply considerations, the art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials. Thus, R & D personnel seek to use alternative feedstocks effectively and selectively to produce light olefins, thereby lessening dependence of the petrochemical industry on petroleum feedstocks. Much attention has been focused on the possibility of using hydrocarbon oxygenates and more specifically methanol or dimethylether (DME) as a prime source of the necessary alternative feedstock. Oxygenates are particularly attractive because they can be produced from such widely available materials as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. The art of making methanol and other oxygenates from these types of raw materials is well established and typically involves the use of one or more of the following procedures: (1) manufacture of synthesis gas by any of the known techniques typically using a nickel or cobalt catalyst in a steam reforming step followed by the well-known methanol synthesis step using relatively high pressure with a copper-based catalyst; (2) selective fermentation of various organic agricultural products and by-products in order to produce oxygenates; or (3) various combinations of these techniques.
Given the established and well-known technologies for producing oxygenates from alternative non-petroleum raw materials, the art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products in order to make an oxygenate to olefin (OTO) process. These light olefin products that are produced from non-petroleum based raw materials must of course be available in quantities and purities such that they are interchangeable in downstream processing with the materials that are presently produced using petroleum sources. Although many oxygenates have been discussed in the prior art, the principal focus of the two major routes to produce these desired light olefins has been on methanol conversion technology primarily because of the availability of commercially proven methanol synthesis technology. Two principal techniques are known in the art for conversion of methanol to light olefins (MTO). U.S. Pat. No. 4,387,263 discloses one MTO processes that utilizes a catalytic conversion zone containing a zeolitic type of catalyst system. The '263 patent reports on a series of experiments with methanol conversion techniques using a ZSM-5 type of catalyst system.
U.S. Pat. No. 4,587,373 discloses using a zeolitic catalyst system like ZSM-5 for purposes of making light olefins. The '373 patent discloses diverting a portion of a methanol feed stream to a DME absorption zone to allow for downsizing of a scrubbing zone. U.S. Pat. No. 5,095,163; U.S. Pat. No. 5,126,308 and U.S. Pat. No. 5,191,141 disclose an MTO conversion technology utilizing a non-zeolitic molecular sieve catalytic material. More particularly these patents disclose using a metal aluminophosphate (ELAPO) and more specifically a silicoaluminophosphate molecular sieve (SAPO) and even more specifically SAPO-34. This SAPO-34 material was found to have a very high selectivity for light olefins with a methanol feedstock and consequently very low selectivity for the undesired corresponding light paraffins and the heavier materials.
The classical OTO technology produces a mixture of light olefins primarily ethylene and propylene along with various higher boiling olefins. Although the classical OTO process technology possesses the capability of shifting the major olefin product recovered therefrom from ethylene to propylene by various adjustments of conditions maintained in the reaction zone, the art has long sought an oxygenate to propylene (OTP) technology that would provide better yields of propylene relative to the classical OTO technology. The driving force for this shift in emphasis towards propylene is the growth rate of the propylene market versus the growth rate of the ethylene market. The existing sources of propylene production in the marketplace are primarily based on conventional steam cracking of naphtha, LPG streams, propane streams and the like. Another principal source of propylene is produced in a fluid catalytic cracking (FCC) hydrocarbon conversion process in the modern day refinery.
US 2003/0139635A1 discloses a fixed bed methanol to propylene (MTP) process for selectively producing propylene from a feedstock of methanol and/or DME. This patent application discloses a flowscheme having an oxygenate to propylene (OTP) synthesis portion having three reactors in a parallel flow arrangement with respect to the oxygenate feed and utilize a steam diluent and fixed beds of oxygenate conversion catalysts. The reactors are connected in a serial flow arrangement with respect to the effluents of the first reactor and the second reactor.
EP-B-1025068 discloses using two reaction zones to convert an oxygenate feed and a by-product fraction containing C4+hydrocarbons to ethylene and propylene. This patent discloses that the two reaction zones allow for independent selection of catalyst and conversion conditions for each zone. This patent discloses using a non-zeolitic molecular sieve catalyst such as SAPO-34 for an oxygenate to light olefin reaction zone and either a non-zeolitic molecular sieve catalyst or a zeolitic catalyst such as ZSM-5 material for the auxiliary reaction zone which operates to convert the C4+ by-product fraction to the desired light olefin (i.e., C2 and C3 olefins). The patent discloses using a circulating fluid bed or a riser reaction for the first reaction zone and a fluid bed or a fixed bed or a fixed tube reactor for the second reaction zone.